In Alzheimer's disease (AD), progressive deposition of amyloid beta-protein (Abeta) fibrils, "amyloid plaques," occurs in the brain. However, neuronal dysfunction may occur prior to plaque formation. Structure-neurotoxicity studies have revealed progressively smaller toxic assemblies, including protofibrils, paranuclei, and ADDLs. In rodents, Abeta oligomers can inhibit long-term potentiation (LTP), a model for learning and memory. In AD patients, Abeta oligomers are present in levels significantly greater than in agematched normal individuals. Biophysical, cell culture, animal, and human studies thus all support the hypothesis that oligpmerization of Abeta is a key event in AD pathogenesis. If so, then the development of therapeutic strategies depends on elucidation of the mechanism(s) of pathologic protein folding, oligomerization, and higher-order assembly. Continuing efforts in our laboratory to understand the earliest steps in Abeta assembly, Abeta monomer folding and oligomerization, have revealed key structural features. These include turn formation in the Val24-Lys28 region of the unstructured Abeta monomer and interactions among the central hydrophobic cluster (Leu17-Ala21), N-terminus, and C-terminus. The four aims comprising this application seek to test mechanistic hypotheses emanating from these observations. Aim 1. To determine the mechanisms of turn formation in the Val24-Lys28 region of Abeta. a. To determine the role of hydrophobic interactions. b. To determine the role of electrostatic interactions. c. To determine the role of amino-acid turn propensity. Aim 2. To determine the mechanisms of intramolecular folding and early Abeta oligomerization. a. To determine the structural dynamics of central hydrophobic cluster (CHC)-C-terminus interactions. b. To determine the structural dynamics of CHC-N-terminus interactions. c. To determine the effects of alternative turn conformations on Abeta monomer structure and oligomerization. Aim 3. To use O?>N acyl migration chemistry to implement a new, quasisynchronous system for studies of Abeta42 folding and self-assembly. a. To synthesize 26-O-acyl-isoAbeta42 (26-AIAbeta42) and study the time-dependent changes in peptide secondary and quaternary structure following initiation of O?>N acyl migration. b. To use quasielastic light scattering spectroscopy to determine kinetic and thermodynamic parameters of Abeta42 self-assembly. c. To use ion mobility spectroscopy-mass spectrometry to monitor early oligomerization events in Abeta self-assembly. d. To synthesize and study the biophysical and biological behavior of Na-protected 26-AIAbeta42. Aim 4. To determine how structural features shown to be critical in controlling Abeta folding and oligomerization affect peptide neurotoxicity.